This invention relates to spark ignited internal combustion engines including apparatus for sensing knock and capable of controlling spark timing or some other engine operating parameter to limit such knock to acceptable levels. It particularly relates to such engines in which knock induces engine vibrations at a plurality of characteristic knock frequencies corresponding to the acoustic cavity resonance modes of the knocking combustion chamber and in which noise vibrations from causes other than knock also occur at a plurality of frequencies.
Knock induced vibrations in such engines are generally sensed as bursts of carrier waves of the characteristic frequencies combined with relative amplitudes determined, for a particular engine combustion chamber, by the acoustic cavity resonance modes and the engine vibration transfer characteristics from the combustion chamber to the sensor location. The envelopes of these vibration waves generally form pulses having time constants corresponding to a much lower frequency. The primary difficulty in reliable knock detection has been in identifying knock induced vibrations in the presence of noise vibrations, which comprise broad band background noise but which may also occur as noise vibration bursts at a frequency which may be the same as one of the characteristic knock frequencies.
Most practical knock detection systems rely at least partly on amplitude discrimination between the peak amplitude of individual knock vibrations in a burst or of the overall burst envelope and an average background noise level. Such systems usually include apparatus to sample the signal and form some sort of average noise level with which the signal itself can be compared. In order to increase the signal to noise ratio, some form of frequency discrimination is usually also employed, with the sensor signal passed through a band pass filter centered on one of the characteristic knock frequencies. It has been recognized that the existence of a plurality of characteristic knock frequencies corresponding to the acoustic cavity resonance modes of the knocking cylinder provides the opportunity for increasing the reliability of knock detection in an engine subject to substantial noise vibrations at one of the characteristic knock frequencies if knock is identified only with the simultaneous occurrence of vibrations at two (or more) of the characteristic knock frequencies. To accomplish this, filtered vibration sensor means may supply vibrations at the characteristic knock frequencies, when they occur, to a pair of signal processing channels, each of which processes the signal at one of the characteristic knock frequencies and signals the presence of vibration amplitude above a reference level. Coincidence sensing means generates a knock signal when the amplitudes on each channel exceed their references simultaneously.
However, even with the system described above, there is a signal to noise ratio problem as the background noise level increases, especially with increasing engine rotational speed. Each channel contains apparatus which compares vibrations at a characteristic knock frequency with some average noise level varying with the average noise vibration level in the engine. As this average noise vibration level increases and approaches the peak levels of the signals to be compared therewith, the reliable detection of knock becomes more difficult due to decreasing signal to noise ratio.